Breakdown of the topological protection by cavity vacuum fields in the integer quantum hall effect

We measure electronic transport at mK temperatures in high mobility GaAs/AlGaAs based heterostructures ultrastrongly coupled to planar circuit resonators. We focus our study on the integer quantum Hall regime. The integer quantum Hall effect is a prototypical topological insulator, where disordered bulk electrons act as an insulator, and the edges are conductive. This system is extremely robust to local defects because of the short range of the disorder potential. This feature allows measuring the quantized resistance in samples with very different geometries and materials. On the other hand, topological protection is not immune to non-local perturbations. Cavity vacuum fields can mediate long-range electron hopping [1], eventually leading to a breakdown of the quantized plateaus.  We experimentally demonstrate the breakdown of topological protection in the integer Hall regime [2]. We show that this effect is most visible at odd filling factors, where the levels are split by the Zeeman energy.  At the same time, the zero field transport is not changed by the cavity and the transport features associated to fractional states are still visible.

[1] C. Ciuti, Arxiv 2107.09435 (2021)

[2] F. Appugliese et al., Arxiv 2107.14145 (2021)